Humus is the unsung hero of the gardening world---the dark, earthy stuff that transforms ordinary topsoil into a rich planting medium. When you think of humus, think of a leaf-strewn forest floor and the dark, dank earth found just below that leafy layer. In topsoil, humus is a naturally occurring organic component found in varying degrees, depending on the terrain. For example, soil found in arid climate zones such as Tucson, Arizona will contain far less humus than soil from forested mountain regions such as Asheville, North Carolina.
Definition of Topsoil
Topsoil is the top layer of soil consisting of minerals and organic materials. It's the place where plants grow their roots and soil-dwelling organisms abound. Topsoil that is low in humus often lacks the proper soil structure to provide a hospitable environment for plants.
Definition of Humus
When organic matter (plant and animal) decomposes over time, it breaks down into the rich, crumbly, black compound known as humus. As defined by the Columbia Encyclopedia, humus is the relatively stable end-product of decomposition, meaning it has broken down as far as it can go. Because it is completely broken down, it is not necessarily nutrient-rich (unlike compost which is organic matter in the process of decomposition). However, the addition of humus to topsoil provides many important benefits.
Benefits of Humus
Whether you are a farmer or a backyard gardener, humus can be used as a soil amendment to improve the physical structure of topsoil and produce higher quality plants. It helps to loosen compact soil and makes it easier to till. Humus improves soil drainage and reduces erosion. The addition of humus to soil increases the soil's ability to hold nutrients and retain water near the plant roots. Also, humus-enriched soil provides better aeration thereby providing oxygen needed by plants to stimulate photosynthesis.
Soil is actually a mini-ecosystem teeming with life---microorganisms such as bacteria and fungi, worms and insects make up part of the diverse soil community. Organisms in soil are necessary to help decompose organic materials thereby releasing nutrients that plants can absorb. Organic matter such as humus provides the fuel these organisms need to perform their role in increasing soil fertility.
According to the Life Sciences Center, crops grown in soil composed of two per cent to five per cent organic humus grow larger and more abundantly with less need for chemical fertilisers.